A network that transfers packets bearing information and provides functions for routing those packets is called a packet network. The packets in question in the present description are IP (Internet Protocol) packets, for example. The present invention applies particularly to a packet network which has a structure conforming to the overlay model, one example of which is represented in FIG. 1 and comprises routers, for example four routers A, B, C, and D, connected to each other by a core transport network 10, for example an optical transport network, including optical cross-connect units OXC1 to OXC4 through which packets processed by each router A, B, C, D can pass in transit. A client-network interface UNI provides logical control between a router and the core network.
More precisely, the present invention applies to a network with an architecture conforming to the overlay model in which the packet domain (for example the IP domain) is relatively independent of the core transport network domain (for example the optical domain). The core network provides connections that are seen from the packet network as point-to-point connections. The packet network acts as a client vis-à-vis the core network. Moreover, the routing protocol of the packet network and that of the core network are independent.
The interface between a router and a cross-connect unit of the core network 10 is a logical control interface that provides for sending signaling messages between an edge router and the optical cross-connect unit of the optical network to which it is connected for the purposes of creating, destroying, modifying, and requesting the status of optical circuits of the optical network. It may be an Optical-User Network Interface (O-UNI) between an IP packet network and an optical transport network, such as those defined by the standardization organizations OIF (Optical Internetworking Forum) and IETF (Internet Engineering Task Force). Under such circumstances, the protocol on which the signaling messages are based is the Resource Reservation Protocol—Traffic Engineering (RSVP-TE).
A packet-switching network generally uses a routing protocol with the essential function of determining the least costly path (also called the shortest path) for packets passing in transit from one router to another, given that a cost, also called a metric, is assigned to each link from one router to another to which it is connected. This applies to an IP network. The routing protocol referred to here is a link state routing protocol, for example of the interior type, such as a protocol of the IGP (Interior Gateway Protocol) link state type, such as in particular the OSPF (Open Shortest Path First) protocol. This is covered by Request For Comments 2328 (RFC 2328) in particular. The routing protocol could equally be the ISIS (Intermediate System to Intermediate System) protocol defined in particular in Request For Comments 3784 (RFC 3784).
The operating principle of an IGP link state type protocol is as follows. Each adjacency of the IP network, which the protocol can determine for itself by means of a particular protocol called the Hello protocol, is associated with a metric of value that represents a characteristic of the adjacency: an arbitrary value assigned by the administrator of the network (for example a value of 1 representing a router that is passed through in transit, the transmission delay on the link concerned, the reliability of the link, etc.). The metric may also be referred to as the cost of the link concerned. The protocol then calculates the routes to all of the routers of the network using a SPF (Shortest Path First) algorithm, the length (or the cost) of a path being determined by summing the metrics of the links forming that path.
The present invention therefore relates only to the packet network which, in an embodiment to be described, is an IP network using a link state routing protocol, for example of the IGP link state type.
The term adjacency refers to a link between two routers that have become neighbors to exchange routing information, i.e. that are neighbors not only because they each have an interface to the same network, but also because they have synchronized their respective topology tables.
The control capacities offered by a link state routing protocol, such as the IGP protocol, are not adequate for the purposes of traffic engineering (TE). Because Shortest Path First (SPF) routing algorithms are optimized on the basis of simple metrics, they fail to take into account traffic and bandwidth availability characteristics in arriving at their routing decisions. Thus congestion can occur if the shortest paths of a plurality of traffic streams converge on the same links or the same routers or if a traffic stream converges on a link or a router that does not have sufficient bandwidth to support it.
The overlay model described above with reference to FIG. 1 solves some of those problems by creating virtual topologies on top of the physical topology of the packet network. A virtual topology consists of virtual circuits that look like physical links to the routing protocol. In this way, traffic and resource control can be envisaged at the level of the virtual topology.
One example of this superposition of a virtual topology is to have the packet network support an optical transport network providing traffic engineering for it. The paper “Distributed Virtual Network Topology Control Mechanism in GMPLS-Based Multiregion Networks”, IEEE Journal on Selected Areas in Communications, vol. 21, no. 8, pp. 1254-1262, October 2003, K. Shiomoto et al. describes a method of dynamically reconfiguring the virtual network topology (VNT) which is distributed (i.e. which includes no centralized coordination in the execution of the reconfiguration of the network) and which consists essentially in the use of a link state routing protocol (of the IGP type referred to above) so that each node shares the same virtual topology and a traffic demand on the individual optical paths, which traffic is measured at the source node. The link state routing protocol is used to disseminate information relating to the virtual topology and information relating to the traffic demand on the optical path. Each node calculates the new topology and compares it with the current topology to identify optical paths that should be activated/deactivated. If that node is the source node of the optical path, the activation/deactivation procedure is applied. When each node has acted in this way, the VNT is reconfigured and the IP traffic is rerouted over the new topology.
The algorithm for calculating the new topology adds new optical paths in order to limit the possibility of congestion, given the traffic, and eliminates any optical path that is under-used. Two traffic thresholds (a higher threshold and a lower threshold) are respectively defined for optical paths that are congested and for optical paths that are under-used. If the traffic demand on an optical path is above the upper threshold, a new optical path is created so that traffic on the congested optical path can be rerouted. The end nodes of the new optical path are selected from all the adjacencies of the end nodes of the congested link. In contrast, if the traffic demand on an optical path is below the lower threshold, that optical path is suppressed, provided that this does not cause congestion.
More precisely, the document referred to here describes a method of dynamic virtual distributed reconfiguration of the topology of Internet Protocol (IP) packet networks supporting MPLS-TE switching or GMPLS switching (Generalized MPLS: see Requests For Comments RFC 3471 et seq.) on wavelength division multiplex (WDM) optical networks. This method is a mechanism which decides to set up or to suppress optical circuits of the WDM network as a function of the traffic load on TE-LSP type paths.
Thus the above document describes the dynamic updating of the topology of a network as a function of the traffic matrix.
The mechanism that is described in the document referred to above nevertheless has the drawback of not offering mechanisms capable of monitoring the impact on the routing protocol of link creation/destruction. In fact, it has been shown that this impact creates topology instabilities and leads to excessively frequent changes. It may even result in congestion or under-use of certain links.